Known schemes for determining time in Global Positioning System (GPS) receivers include demodulating the navigation data message modulated on the satellite carrier signal. However, this approach requires substantial time since the navigation data bit message is transmitted at a relatively low data rate. In NAVSTAR GPS, for example, the navigation data bit message is modulated at 50 bits per second (BPS). Also, the direct time acquisition approach is difficult and often not possible in weak signal environments, for example, in environments where GPS enabled cellular telephones are typically used.
Time determination approaches using a segment of the navigation data, which is known through either infrastructure broadcast or prediction, for correlation against a corresponding signal segment as disclosed, for example, in commonly assigned U.S. Pat. No. 6,532,251 entitled “Data Message Bit Synchronization And Local Time Correction Methods And Architectures” and in commonly assigned U.S. Publication No. 2003/0187575 entitled “Time Determination in Satellite Positioning System Receivers and Methods Therefor”, can operate at relatively low signal to noise ratios. However, these approaches are limited to situations where time is in error by at most a few seconds, and thus these approaches are impractical when time may be in error by more than a few seconds.
In computing a navigation position solution in a satellite positioning system receiver, for example, a NAVSTAR Global Positioning System (GPS) receiver, time must be sufficiently well known to guarantee convergence to the correct solution. Where time is ambiguous, a sufficient number of satellites must be acquired to solve for the time ambiguity.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.